US 7344751 B2
An assembled hematin is formed by depositing hematin on an electrically charged substrate in one or more layers alternating with one or more layers of polyelectrolyte, preferably a cationic polymer. In a method for polymerizing an aromatic monomer, the assembled hematin is contacted with the monomer and a template, preferably an anionic polymer. In a method for polymerizing aniline, the aniline, sulfonated multi walled carbon nano tubes, PEG hematin and a reaction initiator are dispersed in water.
1. A method of forming an assembled hematin, the method comprising depositing one or more layers of hematin and one or more layers of a polyelectrolyte on an electronically charged substrate.
2. The method of
3. The method of
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This application is a DIV of 10/775,579 filed Feb. 9, 2004 now U.S Pat. No. 7,022,420 and is a CIP of 09/994,998, filed Nov. 27, 2001, now abandoned, which claims the benefit of 60/253,109 filed Nov. 27, 2000.
The invention described herein may be manufactured and used by the U.S. Government for governmental purposes without the payment of any royalty thereon.
Recently, there has been an increased interest in tailored development of polyaromatic polymers, particularly polyaromatic polymers that are electrically conductive and/or have useful optical properties. Examples of electrically conductive polymers include certain polyanilines, polythiophenes, polypyrroles, and polyphenols. These conductive polyaromatic polymers may be used in a variety of electronic devices, including electro-chromic devices, light-emitting diodes, electrostatic discharge protection, and light weight batteries. Of these polyaromatic polymers, polyanilines are the most extensively studied, due largely to superior electrical properties, such as high discharge capacity.
In addition to the above-named electrical properties, thermal and structural properties of polyphenols have long been exploited. In particular, phenol-formaldehyde resins, such as novolacs and resols, have found wide application as wood composites, laminates, foundry resins, abrasives, friction and molding materials, coatings and adhesives, fiber binders, and flame retardants. The use of formaldehyde in polyphenol synthesis, however, presents a significant toxicological and environmental hazard.
Despite the industrial utility of polyaromatic polymers, their synthesis remains problematic. Known difficulties in the synthesis of such polymers include inconsistent product composition, due in part to extensive branching of the polymers. In addition, many of the polyaromatic polymers are insoluble, or sparingly soluble, in common solvents, leading to poor processability. The use of toxic reagents, as noted above, is another undesirable feature of current synthetic methods. A search for new methods of synthesizing polyaromatic polymers has not heretofore yielded a commercially viable approach.
Many of the synthetic approaches to forming polyaromatic polymers use a heme-containing enzyme to catalyze the polymerization. Any such catalyst must necessarily be stable and active under acidic conditions, as acidic conditions are required in order to synthesize an electrically conductive form of a polyaromatic polymer, such as polyaniline.
An enzyme suggested for aromatic molecule polymerization is horseradish peroxidase (HRP). Unfortunately, HRP and other peroxidases are inactive at low pH and are prohibitively expensive to use commercially. Hematin has been used to mimic the catalytic activity of HRP. However, despite its lower cost, hematin is a non-ideal catalyst for commercial polymerizations because of its low solubility in acidic, aqueous media. The low solubility of hematin under these conditions leads to a low rate of polymerization and poor yields.
The mechanism for HRP catalyzed polymerization involves the interaction of the heme-iron cofactor of the enzyme with the peroxide, yielding an oxidized heme-iron complex. Subsequently, the oxidized heme-iron complex reacts with the substrate in a one-electron transfer reaction to produce the substrate radical and a new iron-heme complex followed by the coupling of the radicals to form the polymer.
This enzymatic approach has not been extended to polythipophenes or polypyrroles, which have high electrical conductivity. This is because monomers, such as (3,4)-ethylenedioxythiophene (EDOT) and pyrrole (PYR), complexed with the active site of the enzyme catalyst, cause deactivation of the latter and have proved to be unsuitable substrates for this enzymatic polymerization. This deactivation phenomenon drastically limits the prospects for the enzymatic synthesis of a wide range of polymers for possible industrial applications. The present invention evolved from exploration of the possibility of usage of the hydroxy ferriprotoporphyrin Hematin to serve as a catalytic center.
There is a need for a low cost, high efficiency means of synthesizing polyaromatic electronic and photonic polymers, which means is compatible with conditions required to synthesize polymers with commercially desirable properties.
Accordingly, an object of the invention is to provide a derivatized hematin suitable for use as an enzyme catalyst in polymerization of monomers, a method for forming assembled and derivatized hematins, and to methods for polymerizing aromatic monomers with an assembled or derivatized hematin.
Another object of the invention is to provide hematin derivatized with one or more non-proteinaceous amphipathic groups, particularly wherein the amphipathic group is polyethylene glycol.
A further object of the invention is to provide a method for preparing a derivatized hematin by reacting hematin with an amphipathic compound, wherein the hematin is derivatized with an amphipathic compound in the presence of a carboxylic acid activating compound for an aprotic base, particularly wherein the amphipathic compound is soluble over a pH range from about pH 1 to about pH 12.
A still further object of the invention is to provide an assembled hematin which includes alternating layers of hematin and a polyelectrolyte on an electrically charged substrate, particularly wherein the polyelectrolyte is cationic.
A still further object of the invention is to provide a method of forming assembled hematin by alternately depositing one or more layers of hematin and one or more layers of a polyelectrolyte on an electrically charged substrate.
A still further object of the invention is to provide a method for polymerizing aromatic monomers, such as anilines or phenols, more particularly, wherein the polymerization takes place in the presence of a template, preferably anionic.
Still another object of the invention is to provide a method for polymerizing aromatic monomers by contacting an aromatic monomer and a template with an assembled hematin. Preferably, the aromatic monomer is an aniline or a phenol.
A further object of the invention is to provide a method for polymerizing an aromatic monomer, which method includes combining the aromatic monomer with a derivatized hematin catalyst, wherein the hematin preferably is derivatized with polyethylene glycoat (PEG), and the derivatized hematin catalyst and the aromatic monomer are combined with a peroxide to initiate the reaction.
A further object of the invention is to provide a novel method for the synthesis of a conducting complex of polyaniline and multi walled carbon nano tubes (MWCNT) which results in the production of polyaniline/MWCNT which has enhanced electrical and chemical stability, and improved processability.
A still further object of the present invention is to provide a method as described above which results in the synthesis of a polyaniline/MWCNT polymer complex which may be used for applications including but not limited to, nanowires in microchips, high performance nanotubes; reinforced conductive composites; single-molecular transistors, electron emitters for flat panel displays, chemical sensors and artificial muscle actuators.
With the above and other objects in view, the present invention is directed to resolving the current limitations of catalysts used in the commercial synthesis of polyaromatic polymers, by reducing the cost of catalyst and by providing a catalyst that is active and stable over a wide range of pHs. A feature of the present invention is the provision of a method for forming derivatized hematins which are water-soluble and recyclable, virtually eliminating the need for toxic reagents and solvents, thus creating an environmentally friendly synthesis for polyaromatic polymers. Further, the derivatized hematins of the present invention, in combination with a template, reduce the amount of branching during polymerization, leading to structurally more consistent product.
In accordance with a further feature of the invention, there is provided a method for syn-enzymatic polymerization of polypyrroll (PPYR) and/or EDOT in the presence of sulfonate polystyrene (SPS), which results in a novel complex of PPYR and/or poly (3,4)-ethylenedioxythiophene (PEDOT) with SPS, which has exceptional stability, and good processability.
There have been attempts to use different forms of hematin for catalysis, but it was seen that the catalytic activity was incomparably lower than that of the enzyme. It is known to provide for the efficient synthesis of polyaromatic compounds catalyzed by hematin in mixed solvent systems or buffer systems of high pH values. It has been found suitable to use a chemically modified hematin to effectively synthesize conducting polyaniline in the presence of polyelectrolye templates. Work in this area has attempted to manipulate this artificial catalyst towards the synthesis of conducting PEDOT or PPYR, with the ultimate goal of expanding the versatility of this hydroxy ferriprotoporphyrin based catalyst. The method described herein enables the synthesis of such electroactive polymers, suitable for conductive transparent coatings.
In accordance with a still further feature of the invention, there is provided a unique template assisted approach for the synthesis of water-soluble polymers has been found, involving enzymatic polymerization of aniline and phenol with HRP as the catalyst in the presence of an anionic polyelectrolyte. In this case, the polyelectrolyte, such as SPS serves three main functions, namely, to electrostatically align the aniline monomers to promote a para directed approach, to provide counterions for doping the polymer, and to maintain water solubility. Aside from the polyelectrolyte macromolecular templates, micellar templates like sodium dodecylbenzene sulphonic acid, and biological templates, like DNA, have been investigated and seen to be successful nano-reactors in the one-pot enzymatic synthesis of conducting polyanilines. Thus, the template provided an environment wherein the pH and the charge density near the template molecule were different from those of the bulk solution, the polymerization being carried out at pH 4.0, (peroxidases are active in the pH range of 4.0-8.0).
In accordance with another feature of the invention there is provided a novel synthesis of water soluble PEDOT and PPYR using PEG hematin as an efficient catalyst in the presence of SPS as a template. EDOT and PYR have been copolymerized using this unique catalyst.
In accordance with one purpose of the invention, as embodied and broadly described hereinabove, a method for a matrix assisted, syn-enzyme-catalyzed polymerization of aniline comprises the preparation of an aqueous solution containing aniline, sulfonate MWCNT, PEG-Hematin syn-enzyme and reaction initiator (hydrogen peroxide). The procedure is a one-step, in situ reaction, which is highly selective and which produces minimal by-products and chemical waste. The resulting polymer solution can be used immediately as is or purified via such techniques as dialysis and centrifusion and for subsequent processing strategies.
The above and other features of the invention, including various novel details of construction and combinations of steps, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular methods embodying the invention are described by way of illustration only and not as limitations of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.
Reference is made to the accompanying drawings in which are shown illustrative embodiments of the invention, from which its novel features and advantages will be apparent.
In the drawings:
The present invention generally includes a derivatized and an assembled hematin, along with methods of preparing the hematins. The invention also includes methods of polymerizing aromatic monomers in a reaction catalyzed by an assembled or a derivatized hematin.
The methods of the present invention include the use of hematin, a hydroxyferriprotoporphyrin, which has been derivatized with one or more non-proteinaceous amphipathic groups. Examples of amphipathic groups include phosphoglycerides, sphingomyelin, glycolipids, substituted or unsubstituted polyethers and polyalkylene glycols, substituted or unsubstituted polyamines such as polyethyleneimine, polyallylamine, and poly(diallylamine); polyammonium groups, such as poly(allylammonium salts), poly(trimethylallylammonium salts), poly(triethyallylammonium salts), poly(dimethyldiallylammonium salts), poly(diethyldiallylammonium salts), and polysaccharides such as hydroxypropyl cellulose, hydroxymethyl cellulose, and hydroxyethyl cellulose.
Preferred amphipathic groups include polyalkylene glycols, such as polyethylene glycol and polypropylene glycol. Preferably, polyethylene glycol groups have a molecular weight of about 400 to about 100,000, or more, and preferably a molecular weight of about 5,000 to about 15,000.
In one embodiment, the present invention is a method of derivatizing hematin, which includes reacting hematin with one or more amphipathic compounds, thereby forming a derivatized hematin. In a preferred embodiment, the hematin is reacted with one or more amphipathic compounds in the presence of a carboxylic acid activating compound and an aprotic base. In a preferred embodiment, the carboxylic acid activating compound is a dialkylcarbodiimide. In another preferred embodiment, the amphipathic compound is a substituted or unsubstituted polyalkylene glycol. Preferably, the polyalkylene glycol is polythylene glycol.
“Carboxylic acid activating compounds,” as used in the present description, are compounds that serve to couple a nucleophile, such as a hydroxyl, amine, or thiol group, to a carboxylic acid, thereby forming an ester, an amide, or a thioester linkage. Suitable carboxylic acid activating compounds include dialkylcarbodiimides, preferably diisopropylcarbodiimide and dicyclohexylcarbodiimide; N,N′-carbonyldiimidazole; nitrophenol, preferably o-nitrophenol and p-nitrophenol; pentahalophenol, preferably pentachlorophenol, and pentabromophenol; N-hydroxysuccinimide; tosyl chloride; 1-hydroxybenzotriazole; and N-ethyl-N′-(3-dimethylaminopropyl) carbodiimide.
“Aprotic bases,” as used herein, include bases without an exchangeable proton. Suitable aprotic bases include trialkylamines, such as trimethylamine,triethylamine, diisopropylethylamine and triphenylamine; pyridine; pyrimidine; 1,8-diazabicyclo[5.4.0]undec-7-3n3 (DBU); and 1,3,5-triazine.
Derivatized hematins of the present invention can be prepared, for example, by reacting about one-half to about ten mole equivalents of an amphipathic compound, such as polyethylene glycol, with hematin in the presence of an excess of a carboxylic acid activating compound, and an aprotic base, in an aprotic solvent such as dimethylformamide or an ether. The mixture is allowed to stir for about 6 hours to about 6 days, and is then quenched with a large volume of water or other protic solvent. The unreacted reagents are removed by extraction of the reaction mixture with an organic solvent such as ethyl acetate. The water layer is concentrated, preferably by lyophilization, to yield the derivatized hematin.
In another embodiment, the present invention is assembled hematin, which includes one or more layers of hematin alternating with one or more layers of a polyelectrolyte deposited on a substrate. In a preferred embodiment, polyelectrolyte is a cationic polymer, such as a poly(dialkyldiallylammonium salt) or a poly(trialkylallylammonium salt). More preferably, the polyelectrolyte is poly(dimethyldiallylammonium chloride).
In another embodiment, the present invention includes a method of forming assembled hematin, by alternately depositing layers of hematin and a polyelectrolyte onto an electrically charged substrate. Preferably, the polyelectrolyte is a cationic polymer, and more preferably is a poly(dialkyldiallylammonium salt) or a (trialkylallylammonium salt, such as poly (dimethyldiallylammonium chloride).
Assembled hematins of the present invention can be prepared, for example, by dipping a charged substrate, such as a negatively-charged hydrophilized glass slide, into about 0.1 mM to about 100 mM hematin having a pH from about 6 to about 12 at about 0° C. to about 50° C. for about 1 minute to about 100 minutes. The substrate is washed with deionized water and dried with a stream of gas, such as nitrogen or argon. The substrate with a single layer of hematin is dipped into about 0.1 mM to about 100 mM polyelectrolyte having a pH from about 6 to about 12 at about 0° C. to about 50° C. for about 1 minute to about 100 minutes. The substrate is washed with deionized water and dried from a stream of gas, such as nitrogen or argon. The process can then be repeated, from about 1 to about 100 times, to produce multiple alternating layers (or bilayers) of hematin and the polyelectrolyte on the substrate. For a positively-charged substrate, the order of dipping into hematin and a polyelectrolyte is reversed.
In another embodiment, the present invention includes a method of polymerizing an aromatic monomer to form a complex of a polymerized aromatic monomer and a template, by contacting the aromatic monomer and the template with the assembled hematin. Preferably, the template is an anionic polymer, such as poly(styrene sulfonic acid) or a salt thereof. In another preferred embodiment, the aromatic monomer is a substituted or unsubstituted aromatic compound, such as an aniline or a phenol. In yet another preferred embodiment, the complex of the polymerized aromatic monomer and the template forms in solution or the complex forms on the assembled hematin. The complex forming on the assembled hematin can contact one or more layers of hematin or the polyelectrolyte.
Aromatic monomers include substituted and unsubstituted aromatic compounds. Suitable aromatic compounds include 4-(p-hydroxyphenylazo)pyridine and 4-(p-hydroxyphenylazo)pyridinium methiodide. Preferred aromatic compounds for polymerization include aniline, phenol, and 2-methoxy, 5-methylaniline.
Suitable substituents on aromatic monomers will not significantly reduce the rate of polymerization as compared to an unsubstituted aromatic monomer (e.g., will not reduce the rate of polymerization by more than ten-fold). Examples of suitable substituents for aromatic monomers include, for example, halogen (—Br, —Cl, —I, and —F), —OR, —CN, —NO2, —COOR, —CONRR1, —SOkR (where k is 0, 1, or 2), —NRR1, —SR, haloalkyl groups, and —NH—C(═NH)—NH2. R and R1 are independently, —H, an aliphatic group, and aralkyl group, a heteroaralkyl group, and aromatic group, or a substituted aromatic group. A substituted aromatic monomer can have more than one substituent.
Polymerizations catalyzed by assembled hematins of the present invention can be carried out, for example, in a buffered solution, ranging from about pH 1 to about pH 12, at about 0° C. to about 50° C. An aromatic monomer and a template are added to the buffered solution, such that the ratio of aromatic monomer to template repeat unit is about 5 to 1 to about 1 to 5. The concentration of aromatic monomer is about 0.01 M to about 1 M. A quantity of assembled hematin, including about 2 to about 100 bilayers of hematin and polyelectrolyte, is added to the solution. A solution of a peroxide, in an amount sufficient to polymerize the aromatic monomer, is added dropwise over about 5 minutes to about 200 minutes. The reaction is maintained for about 1 hour to about 200 hours. The progress of the reaction can be monitored spectrophotometrically.
A peroxide, as used in the present invention, is an organic or inorganic compound that includes a —O—O— bond, such as ROOR, where R is as defined above. Preferably, one R is hydrogen, to give ROOH. Even more preferably, the peroxide is hydrogen peroxide, HOOH.
Suitable substrates for assembled hematin are any solids that can maintain an electrical charge. Examples of substrates include glasses (e.g., pyrex and glass slides), plastics (e.g., poly(finyl chloride) and poly(ethylene)), ceramics, metals, and the like. Preferred substrates are glass slides, which have been hydrophilized with an aqueous alkali solution, such as Chem-solv, under ultrasonication.
In a preferred embodiment of the present invention, a template is combined with the derivatized hematin, an aromatic monomer, and a peroxide, such that the aromatic monomer aligns along the template and polymerizes to form a complex including the polymerized aromatic monomer and the template. A “template,” as that term is employed herein, refers to a polymer or oligomer that can bind, such as lonically bind, to the aromatic monomer being polymerized.
Suitable template polymers include polyelectrolytes, such as an anionic polymer or a cationic polymer. Anionic polymer templates include polymers that include pendant acid functional groups such as poly(vinylbenzoic acid) and salts thereof, poly(vinyl polyphosphonic acid) and salts thereof, poly(glutamic acid) and salts thereof, poly(aspartic acid) and salts thereof, poly(acrylic acid), and poly(maleic acid co-olefin) and salts thereof. Co-olefins that can be polymerized with maleic acid to form poly(maleic acid co-olefin) include 1-propene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene, 1-nonene, and 1-decene. Preferred anionic polymer templates include poly(styrene sulfonic acid) and salts thereof, lignin sulfonic acid and salts thereof, and dodecylbenzene sulfonic acid and salts thereof.
Optically active templates can be employed in the polymerization method of the invention. When an optically active template is employed, the template can induce macro-asymmetry in the polymerized aromatic monomer due to the close association of the template with the polymerized aromatic monomer in the complex. Examples of optically active templates include polynucleic acids and salts thereof, such as rubonucleic acids and 2′-deoxyribonucleic acids. Other suitable templates include biological receptors, peptides, proteins, zeolites, caged compounds, phenol red, azo compounds, azo polymers, and dendrimers.
In a preferred embodiment, the complex of a polymerized aromatic monomer and a template is a water-soluble complex of a polyaniline and a template. Preferably, the polyaniline (pani) is of the electrically-conducting emeraldine salt form. Emaraldine is an electrically-conducting form of pani, and has a characteristic green color when protonated, or doped.
In another preferred embodiment, the complex including a polymerized aromatic monomer and a template is a water-soluble complex of a polyphenol and a template.
In still another preferred embodiment, a polymerized aromatic monomer complexed to an optically active template has a macro-asymmetry.
A complex of a polymerized aromatic monomer and a template is prepared by contacting an aromatic monomer, such as an aniline or a phenol, and a template with a derivatized hematin in a solution of a pH from about 0 to about 12. Preferably, the solution is buffered, and the pH ranges from about 0 to about 7, and more preferably ranges from about pH 0 to about pH 4. The ratio of aromatic monomer to template (measured as the concentration of template repeat units) can vary from 5:1 to 1:5 (aromatic monomer:template repeat unit), and is preferably from about 2:1 to about 1:2, and is even more preferably about 1:1. A catalytic amount of the derivatized hematin can be added to the reaction mixture either before or after addition of the aromatic monomer. A catalytic amount of the derivatized hematin is typically between about one unit/mL and five units/mL, where one unit will form 1.0 mg purpurogallin from pyrogallol in 20 seconds at pH 6.0 at 20° C.
Preferably, the derivatized hematin is added to the solution after addition of the template and aromatic monomer. In a preferred embodiment, a peroxide is also added to the reaction mixture. The peroxide is added incrementally, such as not to de-activate the derivatized hematin catalyst, until and amount approximately stoichiometric with the amount of aromatic monomer has been added. The reaction can be monitored spectroscopically.
The above polymerization can be carried out in polar solvents such as ethanol, methanol, isopropanol, dimethylformamide, dioxane, acetonitrile, and diethyl ether, but is preferably carried out in water.
The functionalities of the polymers may be tuned to impart requisites, such as sensing, electrochemical, optical and electronic properties through copolymerization with functionalized monomers. The polymers have sites for further modifications, such as covalently coupling other functionalities and even biomolecules through simple coupling chemistry.
The conducting polymers in these polymer complexes allows for use in a wide range of applications including, but not limited to, chemical and biological sensing, electrostatic shielding, corrosion protection, rechargeable batteries, flexible light-emitting diodes, electrochromic devices, smart windows, chaff materials, electromagnetic radiation absorbers and modulaters, and drug delivery systems.
Accordingly, to achieve the foregoing objects and in accordance with the purpose of the invention, as embodied and broadly described herein, a method for matrix assisted, syn-enzyme-catalyzed polymerization or copolymerization of PYR, PEDOT and aniline comprises the preparation of an aqueous solution containing PYR and/or PEDOT, SPS, Hem-PEG syn-enzyme and a reaction initiator (hydrogen peroxide). The procedure is a one-step, in situ reaction, which is highly selective and which produces minimal by-products and chemical waste. The resulting polymers or copolymers solution can be used immediately as is or purified via such techniques as dialysis and centrifugation.
Matrix materials may include, but are not limited to, electrolytes which have various aromatic backbones and/or pendant groups, aliphatic backbones and/or pendant groups, optically active (chromophoric) backbones and/or pendant groups, electrically active backbones and/or pendant groups and various degrees of sulfonation/functionalization. The ionized groups on these electrolyte matrices may include but are not limited to sulfonates, carboxylates, phosphates, and borates. Manipulation of the molecular weight and purity of the matrices allows for optimized polymerization and processing conditions.
The present invention further relates generally to a syn-enzymatic polymerization process of aniline in the presence of sulfonated multi wall carbon nano tubes (MWCNT) which results in a novel complex of polyaniline (pani) and MWCNT which has exceptional stability, and good processability.
Carbon nanotubes (CNT) have been the focus of extensive research since their discover. Several investigators have reported unique physical and mechanical properties for this form of carbon. CNT exhibit electronic properties with thermal conductivities higher than diamond and present significant opportunities for the development of novel multifunctional material systems. The mechanical properties of CNT, such as stiffness, strength and resilience, surpass those of most known materials. The combination of low density and high strength may also lead to the development of high performance nanotube reinforced composites. Nanotubes and/or nanowires have been shown to have application in electronic nanoscale devices, such as single-molecular transistors. Other potential applications are field emission electron emitters for flat panel displays, chemical sensors and artificial muscle actuators. There has been considerable scientific interest in the development of electrically conducting molecular wires involving these nanotubes.
Pani has been one of the most popular conducting polymers due to its low cost and excellent electronic and environmental stability. The applications of pani have been somewhat limited due to poor processability/solubility. However, the use of polyelectrolytes as templates for the preferential alignment of monomer molecules, along with providing charge compensation (doping) in the polymer, has alleviated most of these problems. It has been demonstrated that templates, like polystyrene sulfonate, polyvinyl phosphonic acid, and biological templates, like DNA, can be used for the horseradish peroxidase (HRP) catalyzed synthesis of conducting water-soluble pani. Recently, a synthetic enzyme based on a hydroxy ferriprotoporphyrine hematin covalently bound to PEG, was found to be a cost-effective alternative to HRP. This synthetic enzyme, known as PEG-Hematin, was found to be more robust and allowed the flexibility to work over a broad range of pH, in contrast to HRP. It has been found that sulfonated CNT may be used as a template for the PEG-Hematin catalyzed polymerization of aniline. By doing so, one can produce nanowires comprised of conducting pani covering the CNT with dimensions on the order of tens of nanometers in diameter and microns in length.
The impetus behind the novel synthetic approach described herein is the use of an electrolyte matrix during the polymerization process. Here, the electrolyte serves three critical functions. First, the electrolyte (sulfonated MWCNT) serves as a matrix upon which the aniline monomers preferentially align to promote the head to tail coupling and extended conjugation of the resulting polymer chains. Secondly, the electrolyte acts as a large molecular dopant species which is complexed and essentially “locked” to the pani chains. Current limitations to the actual use of pani and in electronic and optical applications have been due to poor dopant stability where the small ionic dopants or chromophores that are currently used are known to diffuse away with time and/or conditions and thus result in a loss of conductivity and/or optical activity. This “locking” of a large sulfonated MWCNT dopant to the pani chains is significant in that it ensures maintainment of the polymers “doped” form and hence stability of the desired electronic and optical properties. Lastly, the electrolyte template serves to promote mechanical stability of the final MWCNT/Pani complex. Since panis are known to be virtually unprocessable without harsh, chemical modification or involved synthetic strategies, this new approach provides unsurpassed environmentally compatible, facile, and inexpensive processing opportunities for real device fabrication.
The MWCNT sulfonate in these complexes allows for a wide range of applications including but not limited to nanowires in microchips, high performance nanotubes; reinforced conductive composites; single-molecular transistors, electron emitters for flat panel displays, chemical sensors and artificial muscle actuators.
The present invention is premised on the discovery that unsurpassed electrical and optical stability, processability, tunability and environmental compatibility are imparted to a new matrix assisted syn-enzymatic polymerization of aniline and phenol. In addition, with judicious choice of matrix and/or monomer, the final polymer complex properties may be tailored to suit a wide range of real device applications.
The present invention will now be further described by the following non-limiting examples.
Synthesis of PEG-Hematin complex
The PEG-hematin complex was obtained through the coupling of PEG chains to a hematin molecule through ester linkages as shown in
The complex was characterized using NMR and FTIR spectroscopy. The average extent of modification of the acidic groups of hematin was determined using UW-vis spectroscopy. The UV-vis spectra of the PEG-hematin exhibited a decrease in the Soret band (420 nm), a porphyrin centered π-π* transition, in comparison to hematin, which was used to calculate the amount of hematin present in the sample. However, the energy and spectral bandwidths of PEG-hematin were indistinguishable from hematin, which indicated that the modification of hematin by poly (ethylene glycol) does not affect the heme structure. Based on this assumption, the average concentration of hematin in the PEG-hematin sample was subsequently determined to be 67% by weight.
An FTIR spectrum of PEG-hematin indicated the presence of an ester functionality by the appearance of a doublet at 1646 and 1651 cm−1 (similar to diethyl phthalate) accompanied by the complete disappearance of the peak at 1712 cm−1 for the acid carbonyl of hematin (
An 1H NMR spectrum of PEG-hematin in DMF-d7 shows the disappearance of the peak at 10.2 ppm, which was assigned to the carboxylic proton of hematin (
The activity of the PEG-hematin was assessed through the oxidation of pyrogallo (0.5%) to purpurogallin in 14 mM potassium phosphate buffer in the presence of 0.027% (w/w) hydrogen peroxide. The activity of the PEG-hematin was found to be approximately 30-fold higher as compared to native hematin at a pH 4.0 (
Synthesis of Polyanine
The polymerization of aniline was carried out in 0.1 M sodium phosphate buffer (10 mL) maintained at pH 1. To this buffer solution the aniline monomer was added. The catalyst, PEG-hematin (60 μg), was added only just prior to the addition of hydrogen peroxide. The polymerizatrion was initiated by the incremental addition of a stoichiometric amount of hydrogen peroxide, with respect to aniline. 0.3% H2O2 (w/v) was used with constant stirring and the progress of the reaction was monitored spectroscopically (
This reaction thus proved the versatility and ability of the PEG-Hematin for the synthesis of stable conducting pani even in the absence of template. The pani formed in this case was again redox reversible as proved by cyclic voltammetry studies.
Synthesis of Sodium Poly (sodium-4-styrenesulfonate)-Polyaniline Complex
The polymerization of aniline was carried out in 0.1 M sodium phosphate buffer over a range of pH conditions from pH 1-4. A 17 mM solution of SPS template in phosphate buffer (100 mM) was prepared to which the aniline monomer was added in a 1:1 molar ratio of aniline to sodium styrene sulfonate monomer. The catalyst, PEG-hematin (5 mg), was added just prior to the addition of hydrogen peroxide. The polymerization was initiated by the incremental addition of a stoichiometric amount of hydrogen peroxide (relative to aniline). In all cases, 0.3% H2O2 (w/v) was used with constant stirring, and the progress of the reaction was monitored spectroscopically. On completion of polymerization, the solution was transferred to individual regenerated natural cellulose membrane bags (molecular weight cut-off 10,000 D) and were dialyzed against 5000 mL of acidified deionized water maintained at pH 4.0 to remove unreacted monomers and oligomers. The solid SPS-Pani complex was obtained by evaporation of the deionized water followed by drying in a vacuum oven.
It was observed that the solution slowly turned dark green, indicating the formation of the doped emeraldine salt form of conducting pani. The UV-vis absorption spectra of the Pani/SPS complex, formed at different time intervals over a period of 2 hours at pH 4.0 after initiation of polymerization reaction, is shown in
The redox tunability of the pani formed was further demonstrated by dedoping the emaraldine salt form of pani at high pH and then redoping with acid. With increasing pH (dedoping) on titration with 1 N NaOH, the polaron bands at 400 nm and 8(0 nm were found to diminish, while a new band at 600 nm began to emerge due to the exciton transition of the quinoid ring giving rise to a blue solution indicating that the Pani had been fully dedoped to the base form. On titrating the solution back with 1 N HCl (redoping), a reversible color change was observed and the spectra is shown in
The conductivity of the emeraldine salt form of pani synthesized at pH less than 4 was found to be about 10−3 S/cm.
Furthermore, cyclic voltammetry studies were carried out to determine the electrochemical nature of pani synthesized by the PEG-hematin catalysis. The cyclic voltammogram of a cast film of an SPS-pani complex (
Synthesis of Lignosulfonate-Pani Complex
5.2 mg of a lignin sulfonate polyelectrote complex was dissolved in 10 mL of sodium monophosphate buffer (0.1 M) maintained at pH 4.0. This was followed by the addition of 18 μL of aniline, a catalytic amount of PEG-Hematin and an amount of hydrogen peroxide (0.3%) stoichiometric with aniline. The reaction mixture was allowed to stir until precipitation of the polyelectrolyte-Pani complex ceased. The reaction was also carried out in solutions having pHs ranging from pH 1-4 (
When the polymerization was conducted at pH 3.0, there was a peak of low intensity at 767 nm for the emeraldine form of polyaniline, which was completely absent during polymerization at pH 4.0. The extended absorption of 1200 nm indicated the formation of the extended conjugation of the pani backbone. Thus, the synthesis of pani complexed with a natural polymer further widens the scope of applications to other natural polyelectrolytes to form versatile, environmentally benign conducting polymers.
Synthesis of DNA-Pani Complex
The polymerization of aniline in the presence of Calf Thymus DNA was carried out in sterile 10 mM phosphate buffer. A 1.0 mM calf thymus DNA solution was prepared by dissolving the required amount of DNA in 10 mL of sterilized sodium phosphate buffer maintained at pH 4. The concentration of DNA was determined by the UV absorbance at 258 nm. To this DNA solution, 4.5 μl (5 mM) of aniline was added. The pH of the solution was again checked and adjusted to 4.3, and 5 mg of PEG-Hematin were added. To this reaction mixture, a solution of hydrogen peroxide (0.3% solution, 4.5 μl, 5 mM) was added drop-wise, to initiate the polymerization and reaction of aniline was followed using UV-Vis spectroscopy and circular dichroism polarimetry.
When the aniline monomer was added to a DNA solution at pH 4.3, the electrostatic interaction between the protonated aniline monomers and the phosphate groups in the DNA caused the monomers to closely associate with the DNA. The association of the protonated aniline monomer on the DNA template facilitated a predominantly para-directed coupling and inhibited parasitic branching during the polymerization. The high proton concentration around the phosphate groups also provided a unique local lower pH environment that permitted the polymerization of aniline at a higher pH than that necessary with conventional chemical polymerization of aniline. The polymerization was catalyzed by PEG-hematin and initiated by hydrogen peroxide. However, as the polymerization proceeded over a period of time and a critical chain length was attained, the DNA-Pani complex precipitated out of solution. It was concluded that the complex remained soluble as long as there were enough phosphate groups on the DNA available for solvation. As the polymerization proceeded, the preferred molecular interaction between the charged aniline groups and the phosphate groups of DNA caused the growing chain to occupy a majority of these sites leading to the salting out of the DNA-Pani complex. The polymerization reaction was followed using UV-vis spectroscopy and circular dichroism polarimetry. The UV-vis spectra of the DNA-Pani complex recorded after initiation of the polymerization are shown in
The bases of the nucleic acid have a plane of symmetry and thus are not intrinsically optically active. However, the deoxyribose sugar is asymmetric and since the bases are attached to the anomeric carbon of these sugars, the sugar can induce a circular dichroism in the absorption bands of the bases. These bands may be observed either for the intensely electronically allowed π-π* transitions, or for the weakly allowed n-π* transitions because these transitions are magnetically allowed. Also, the π electron systems of the bases make them hydrophobic, so the bases tend to stack in hydrogen-bonding solvents to minimize the π-electron surface area exposed to the solvent. The hydrophobic planes and hydrophilic edges as well as charge-charge interactions cause the bases to stack and the polymer to adopt a helical structure. Preferential handedness is induced in these helical structures by the intrinsically asymmetric sugars, giving the DNA polymer a whole super asymmetry. The electronic transitions of these chromophoric bases are in close proximity and can thus interact to give well-defined CD spectra. The CD spectrum of the DNA-Pani complex showed a reduction in the intensity of the peak at 275 nm (
Synthesis of Poly(2-methoxy, 5-methylaniline)-SPS Complex
The polymerization of 2-methoxy, 5-methylaniline (2M5M) was carried out in 0.1 M sodium phosphate buffer of pH 4.0. A 17 mM SPS template solution, as measured from the concentration of sodium styrene sulfonate monomers, in phosphate buffer (10 mL) was prepared, to which 2M5M (24 mg) was added in the desired (1:1, 2M5M:SPS) molar ratio. The polymerization was initiated after addition of 5 mg of PEG-Hematin, by the incremental addition of an amount of peroxide (0.3% w/v) stoichiometric with 2M5M, with constant stirring. The progress of the reaction was monitored spectroscopically. After the reaction was complete, the solution was dialyzed to remove the unreacted monomers, followed by evaporation to yield a SPS-poly(2M5M) complex.
The UV-vis absorption spectra of the poly(2M5M)/SPS complex formed is shown if
Synthesis of Sodium Dodecylbenzenesulfonic Acid-Pani Complex
Polymerization of aniline was carried out in 0.1 M sodium at pH 4. At 17 mM solution of dodecylbenzesulfonic acid (DBSA) in phosphate buffer (100 mM) was prepared to which the aniline monomer was added in the desired (1:1, Aniline:DBSA) molar ratio. The catalyst, PEG-Hematin (5 mg), was added just prior to the addition of hydrogen peroxide. The polymerization was initiated by the incremental addition of an amount of hydrogen peroxide stoichiometric to aniline. In all cases, 0.3% H2O2 (w/v) was used with constant stirring. The progress of the reaction was monitored spectroscopically.
Synthesis of SPS-Polyphenol Complex
A polymerization reaction was carried out in 10 mL of aqueous phosphate buffer (100 mM). The pH of the reaction media for the phenol polymerization was maintained at pH 7.0 and equimolar concentrations (17 mM) of SPS, with respect to the concentration of the repeat units, and phenol monomer were added to the buffered solution, followed by 10 mg of the PEG-hematin. The reaction was initiated by addition of a stoichiometric, with respect to phenol, amount H2O2 (30% w/v) in one lot to facilitate the formation of high molecular weight polypenol. The reaction was monitored spectroscopically. A control experiment was also carried out simultaneously in the absence of catalyst. The final products were dialyzed using Centricon concentrators (10,000 Mw cut off, Amicon Inc., Beverly, Mass.) to remove unreacted monomers. The samples were then dried under vacuum at 50° C. and used for further analysis. The yield was calculated to be typically 95% or higher.
The PEG-hematin complex was also found to catalyze the polymerization of phenol at pH 7.0 more efficiently than that compared to the native hematin and peroxidase (
Preparation of Assembled Hematin
Glass slides (25 by 75 mm) were hydrophilized with 1% Chem-solv solution in deionized water under ultrasonication for use as substrates. This treatment generates negative charges on the surface of the slides due to partial hydrolysis. After 3 hours, the slides were ultrasonicated twice in deionized water for 30 minutes before use.
The electrostatic layer-by-layer deposition process was carried out in two steps. PDAC (10 mM) and hematin (3 mM) solutions were prepared over a pH range from 5 to 11. In the first step, hydrophilized glass slides were immersed in PDAC solution for 10 minute at room temperature and washed with deionized water for 5 minutes. After the deposition and washing steps, the slides were dried with a stream of nitrogen. In the second step, the substrates with a single layer of PDAC were immersed into the hematin solution for 10 minutes and subsequently washed with deionized water and dried with a stream of nitrogen to produce an assembled hematin, having a bilayer film of PDAC/hematin. This dipping procedure was iterated to build up multilayer films.
Synthesis of Pani-SPS Complex Using Assembled Hematin
Polymerization of aniline was carried out at room temperature in a 40 mL, 0.1M phosphoric acid buffer solution, which contained a 1:1 molar ratio of SPS (MW 1,000,000; moles correspond to quantity of monomers units) to aniline 0.167 g (0.81 mmol). SPS was added first to the buffered solution, followed by an addition of 2.1 mL of aniline stock solution (0.036 mL aniline to 1 mL buffer at pH 1.4) with constant stirring. A seventeen bilayer Hematin/PDAC assembly was immersed in the solution. To initiate aniline polymerization, 11 mL of 0.25% H2O2 was added dropwise, incrementally, over 30 minutes. The reaction was maintained for 24 hours, and carried out at different pH values (1.0, 2.0, 3.0). The rate of assembled hematin catalyzed polymerization was monitored by a Perkin-Elmer Lamda-9-UV-vis spectrophotometer at room temperature.
Synthesis of Aniline Monomer in Presence of MWCNT
Aniline monomer polymerizes in the presence of sulfonated multiwall carbon nano tubes (MWCNT), an oxidizing agent which is comprised of syn-enzyme (Hematin-PEG) and an electron acceptor (hydrogen peroxide) to give a MWCNT sulfonate/pani complex which is dispersed in water. The pani as synthesized may be simultaneously oxidized at higher pH's than can be done using prior art techniques, resulting in the emeraldine “conducting” form and undergoes reversible oxidation and reduction with change of pH.
Synthesis of 2-Methoxy, 5-Methylaniline in presence of MWCNT
2-methoxy, 5 methylaniline polymerizes in the presence of sulfonated MWCNT, an oxidizing agent which is comprised of syn-enzyme (Hematin-PEG) and an electron acceptor (hydrogen peroxide) to give a MWCNT sulfonate/pani complex which is dispersed in water. The pani as synthesized may be simultaneously oxidized at significantly higher pH's than can be done using prior art techniques, resulting in the emeraldine “conducting” form and undergoes reversible oxidation and reduction with change of pH.
Synthesis of Phenol Monomer in Presence of MWCNT
Phenol monomer polymerizes in the presence of sulfonated MWCNT, an oxidizing agent which is comprised of syn-enzyme (Hematin-PEG) and an electron acceptor (Hydrogen peroxide) to give a MWCNT sulfonate/polyphenol complex which is dispersed in water. This polyphenol complex may be an environmentally friendly, cost effective, and more processable substitute for current polyphenolic resin materials.
This invention provides a significant advancement over current methods used for the synthesis of a conducting form and processable form of polyaniline. This approach addresses and resolves processability and stability of the current limitations of the commercial use of polyaniline. The syn-enzymatic synthesis provides a specific, simple (one-step) and environmentally friendly synthetic approach, while the MWCNT provides mechanical stability and processability. In addition, since MWCNT shows extremely interesting applications in liquid display and already used for commercial applications, the MWCNT/polyaniline complex described in this invention is expected to transition effectively into many of these already established applications where electrical activity and/or conductivity is desirable. Examples of such applications include chemical and biological sensoring, electrostatic shielding, corrosion protection, light weight rechargeable batteries, flexible light-emitting diodes, electrochromic devices, smart windows, chaff materials, radiation absorbers for optical illusions, nanowires in microchips, high performance nanotube; reinforced conductive composites; single-molecular transistors, electron emitters for flat panel displays, chemical sensors and artificial muscle actuators.
While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.